Downlink Multi-user, multiple-input-multiple-output (DL MU-MIMO) operation is being defined by the Institute for Electronic and Electrical Engineering (IEEE) in the 802.11ac standard. With DL MU-MIMO a transmission comprising multiple spatial streams may have different subsets of the streams directed to different users. Two transmit modes are possible: the first is referred to as resolvable LTFs; and the second referred to as non-resolvable LTFs. With resolvable LTFs (long training fields) each user sees training for all spatial streams, including the streams directed toward other users. With non-resolvable LTFs a particular user only sees training directed to that user and as a result the LTFs overlap in time.
Thus, a critical need is prevalent for frame format techniques for non-resolvable long training fields in wireless networks.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.